This invention relates to a high-performance photocathode for use in pickup tubes such as television camera tubes and image intensifier tubes.
It is known to construct a photocathode having the following main components:
a so-called window layer consisting of P.sup.+ type semiconductor material in which the forbidden band is of sufficient width to ensure that said layer is transparent to the wavelengths of the light to be detected and which is bonded to a glass wall for receiving the light to be detected; PA1 a so-called absorption layer consisting of P.sup.+ type semiconductor material in which the forbidden band is of sufficiently small width to convert the light photons to be detected into electron-hole pairs; PA1 a so-called emission layer consisting of material which produces negative electron affinity at the end of the absorption layer in order to emit into vacuum the electrons which are released within the absorption layer.
The maximum detectable wavelength is limited by the width of the forbidden band of the material which constitutes the absorption layer. By applying a positive bias to said layer at the end opposite to the window layer, it is possible to employ materials which have a small forbidden bandwidth while maintaining good emission efficiency and it is therefore possible to detect light having longer wavelengths. A bias can be applied to the absorption layer by means of a connection with said layer or by a very thin metallic electrode interposed between said layer and the emission layer. A photocathode of this type is described in the article by J. J. Escher et al., IEEE-EDL2, 123-125 (1981).
The performance of a photocathode of this type is limited in particular by the characteristics of the absorption layer. The thickness of this layer is in fact determined by making a compromise between on the one hand high absorption of photons of the light to be detected, which requires a thickness as great as possible, and on the other hand high efficiency of transmission of electrons as well as a low dark current, which calls for minimum thickness of the absorption layer, also in order to obtain two-dimensional quantization of energy levels of electrons and holes in the plane of the sublayers.
As a general rule, the thickness of this layer is of the order of 1 micron. This permits good efficiency of transmission of electrons but is not sufficient to absorb all the photons of the light to be detected, especially the photons corresponding to the longest wavelengths. The object of the invention is to provide a photocathode which offers higher efficiency than photocathodes of known types. The invention is accordingly directed to a photocathode having an absorption layer constituted by a plurality of individual sublayers so as to achieve at the same time very good absorption of photons, high efficiency of transmission of electrons released by the photons and a low dark current.